Films which are transparent in the visible spectrum and have electrical conductivity are generally used as a transparent electrode in new display systems, such as liquid crystal, electrochromic, and electroluminescence display systems. They are also used in transparent materials to prevent static buildup, as shielding from static electricity and electromagnetic waves, as well as for static recording.
Transparent electrically conductive films composed of a stannic oxide film or indium oxide film on a glass sheet are known, but they find only limited utility because of the low flexibility and processability of the glass base conventionally used. As an alternative, plastic-based transparent electrically conductive films have attracted increased interest because of their light weight and high flexibility, processability, and impact resistance. However, since plastic base is not heat-resistant, the new transparent electrically conductive films cannot be produced by the same method as has been used to manufacture the glass-based film.
The conventional process for the production of the glass-based film comprises the spraying of a solution of tin tetrachloride in hydrochloric acid onto a glass sheet heated to several hundred degrees Celsius, followed by oxidation at high temperature to form a thin film of stannic oxide. A recently developed method uses indium oxide as an evaporation source which is deposited as a vapor on a glass sheet (heated normally to about 300.degree. to 350.degree. C.) in a high degree of vacuum in the range of from 10.sup.-4 to 10.sup.-5 mmHg. Obviously, either method requires the heating of the glass base to high temperatures, as does another known method which supplies the vacuum system with water vapor or a gas that contains water vapor. The last mentioned method aims at forming an electrically conductive film of low resistivity, say 100 ohms/cm.sup.2 or less, but even that method requires heating of the glass base to a temperature between 300.degree. and 350.degree. C. In short, the heating of the glass base is indispensable to the manufacture of a desired glass-based electrically conductive film in the conventional technique, and apparently, such technique is not applicable to the production of a thin electrically conductive film using a plastic base.
Several methods have been proposed to solve the stated problem, and most of them rely on a vacuum vapor deposition process using either indium oxide or indium as a primary evaporation source.
Examples of the method using oxide indium as an evaporation source are described in Japanese Patent Publication Nos. 35431/76 and 37667/76 wherein an indium oxide vapor is deposited on a plastic base in a high degree of vacuum (i.e., less than 1.times.10.sup.-3 mmHg, typically between 1.times.10.sup.-4 to 1.times.10.sup.-5 mmHg) that was either unheated or heated to a suitable temperature tolerable to the plastic base, followed by oxidative treatment under heating primarily in an oxidizing gas atmosphere. But this method demands relatively severe conditions for the oxidation treatment that follows the vacuum deposition of oxide indium vapor; for example, the optimum temperature for heat treatment in air for a practical period is between 200.degree. and 250.degree. C. or higher. During vacuum deposition, indium oxide is decomposed to lower oxides, so the purpose of oxidative treatment is to convert them into higher oxides, but since the conditions for that treatment are severe, as mentioned above, only limited types of plastics can be used as the base. In addition, the method that uses an oxide as an evaporation source must heat the source to a temperature higher than 1,300.degree. C., and typically between about 1,500.degree. and 2,100.degree. C. This requires the use of a heat-resistant expensive crucible as a container for the evaporation source. Furthermore, the distance by which the evaporation source is conventionally separated from the plastic base is such that there is a great possibility that the plastic base will be intensely heated by radiation, and this is another limiting factor on the material used as the plastic base.
The other method, that uses indium as an evaporation source, deposits the vapor of indium oxide on a plastic base that is formed by oxidizing indium in a vacuum system having relatively low degree of vacuum (i.e., ca. 2.times.10.sup.-2 to 1.times.10.sup.-4 mmHg) which is being supplied with an oxidizing gas. Japanese Patent Publication No. 14304/65 describes a method that facilitates the conversion of indium to indium oxide by heating the plastic base to at least 100.degree. C., and typically between about 110.degree. and 150.degree. C., before vacuum deposition. Japanese Patent Publication No. 8137/68 describes a method wherein a thinner film of vapor deposit is provided by performing deposition at a rate higher than 16 A/sec, typically higher than 50 A/sec, and after the deposition, the film is held at a temperature of around 100.degree. C. for several hours to promote the conversion of indium to an oxide form.
In these methods wherein the plastic base is not heated at all or is heated to relatively low temperatures, and wherein the temperature of the evaporation source is not great, only a small amount of heat is radiated from the evaporation source. Even the temperature at which the indium oxide film is held after its deposition is well tolerated by the various types of plastic base.
As discussed above, almost all types of plastic bases can be used in the method that uses indium as an evaporation source, and hence, the method has been considered the most desirable process for producing a transparent electrically conductive film on an industrial scale. But in spite of that great advantage, the method cannot provide an electrically conductive film having good characteristics, and in particular, its transparency is not completely satisfactory. For example, the film produced according to the method described in Japanese Patent Publication No. 14304/65 has a transmittance of visible light (600 nm) of only up to about 30%, even if it is very thin. Some improvement in transparency can be obtained in the method of Japanese Patent Publication No. 8137/68 by performing suitable treatments, such as oxidative treatment under severe conditions, but the maximum transmittance of visible light through a thick film (e.g., 1,000 A or more) is about 80%, and a higher transmittance cannot be achieved.